Wet strength is a desirable attribute of many disposable paper products, such as napkins, paper towels, household tissues, disposable hospital wear, etc. which come into contact with water in use. However, an untreated cellulose fiber assemblage will typically lose 95% to 97% of its strength when saturated with water. To improve the wet strength paper manufacturers in the past have added wet strength resins during the paper making process.
These wet strength additives have typically resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Permanent wet strength in paper products is often an unnecessary and undesirable property. Paper products such as toilet tissues, etc., are generally disposed of after brief periods of use into septic systems and the like. Clogging of these systems can result if the paper product permanently retains its hydrolysis-resistant strength properties.
More recently, manufacturers have added temporary wet strength additives to paper products for which wet strength is sufficient for the intended use, but which then decays upon soaking in water. Decay of the wet strength facilitates flow of the paper product through septic systems.
As recognized in the pertinent literature relating to wet strength of paper products, such as Stannet, "Mechanisms of Wet Strength Development in Paper," Surface Coatings Related Paper Wood symposium, pp. 289-299 (1967) and Westfelt, "Chemistry of Paper Wet Strength. I. A survey of "Mechanisms of Wet Strength Development," Cellulose and Chemistry and Technology, Vol. 13, pp. 813-825 (1979), paper products develop dry strength in part due to interfiber hydrogen bonding. When the paper product is wetted, water disrupts the hydrogen bonds and, as a consequence, lowers the strength of the paper product. Historically, wet strength of paper products has been increased primarily by two approaches. One approach is to prevent water from reaching and disrupting the hydrogen bonds, for example, by coating the paper product. Another approach is to incorporate additives in the paper product which contribute toward the formation of interfiber bonds which are not broken or, for temporary wet strength, which resist being broken, by water. The second approach is the technique of choice, especially for tissue products. In this latter approach, a water soluble wet strength resin is added to the pulp, generally, before the paper product is formed (wet-end addition). The resin generally contains cationic functionalities, so that it can be easily retained by the cellulose fibers, which are naturally anionic.
A number of resins have been used or disclosed as being particularly useful for providing wet strength to paper products. These include urea-formaldehyde and melamine-formaldehyde resins. Such resins have limited wet strength decay. Polyamide-epichlorohydrin resins have also been used in paper products. However, they also provide little wet strength decay.
Numerous approaches for providing paper products claimed as having good initial wet strength which decays significantly over time have been reported.
Resins formed by reacting glyoxal (CHOCHO) with water-soluble vinylamide polymers are suggested as possessing temporary wet strength in U.S. Pat. No. 3,556,932, Coscia et al., issued Jan. 19, 1971. However, wet strength decay is reported only at alkaline pH levels--conditions not necessarily present in septic systems. Also, papers incorporating such resins lose only about half their wet strength upon exposure to water. Greater degrees of wet strength decay are desirable.
U.S. Pat. No. 3,740,391, Williams et al., issued June 19, 1973, describes a water-soluble thermosetting wet strength agent for paper which is prepared by reacting an amidated ionic glucopyranosyl compound with glyoxal. Paper products containing this wet strength agent lose about one half their original wet strength after soaking in water for 24 hours. Such wet strength decay is not sufficiently rapid since the paper products in which the resin is used, such as toilet paper, are generally disposed of within a few minutes use.
U.S. Pat. 4,605,702, Guerro et al., issued Aug. 12, 1986, discloses temporary wet strength resin made by reacting a vinylamide polymer with glyoxal, and then subjecting the polymer to an aqueous base treatment. The product is said to provide tissue paper which loses a part of its wet strength when soaked in water at neutral pH.
U.S. Pat. No. 4,603,176, Bjorkquist and Schmidt, issued July 29, 1986, discloses temporary wet strength resins made by reacting glyoxal with a cationic vinylamide copolymer. The cationic vinylamide copolymer is prepared from a nonnucleophilic, water-soluble monomer, such as dimethylacrylamide, an acrylamide nucleophilic monomer, and monomer containing a quaternary nitrogen. Subsequent to the reaction of glyoxal with the nucleophilic amide, the final product has from about 3% to about 65%, mole percent basis, of monomeric units with nucleophilic amide functionalities and about 1% to about 20%, mole percent basis, of monomeric units with glyoxal-substituted amide functionalities.
Modified starch temporary wet strength agents are marketed by the National Starch and Chemical Corporation (New York, N.Y.). This type of wet strength agent can be made by reacting dimethoxyethyl-N-methyl-.alpha.-chloracetamide with cationic starch polymers. Modified starch wet strength agents are also described in U.S. Pat. No. 4,675,394, Solarek, et al., issued June 23, 1987. Unfortunately, such wet strength agents typically deliver relatively high dry strength in conjunction with the level of wet strength provided. This is undesirable for products, such as tissue paper, for which softness is important because increased dry strength generally is accompanied by decreased softness.
U.S. Pat. No. 3,410,828, Kekish, issued Nov. 12, 1968 and its parent, U.S. Pat. No. 3,317,370, Kekish, issued May 2, 1967, disclose wet strength resins which comprise water soluble copolymers of an aldehyde monomer, such as acrolein, and a nitrogen heterocyclic monomer containing an ethylenically unsaturated group capable of polymerization with aldehydes. The advantages of these copolymers, as set forth in their respective patent disclosures, is that retention aids (such as aluminum salts), are not needed because the copolymers have no anionic character. Optionally, the copolymers can be made cationic by reacting a water soluble amine or quaternary amine with copolymers. The disclosures of these patents generally link increased molecular weight with increased effectiveness without consideration of the particular properties and characteristics desirable for temporary, as opposed to permanent, wet strength.
In spite of the technology described above, there exists a continuing need to provide temporary wet strength resins having increased wet strength decay rates.
U.S. Pat. No. 3,096,228, Day et al., issued July 2, 1983, describes paper products wherein a solution of glyoxal is applied to a paper web. Upon exposure to water, this paper described as being is able to resist a rupture for only about a minute and as disintegrating within about 3 minutes. While this approach using glyoxal monomers provides rapid wet strength decay, it does not provide paper products with sufficient initial wet strength or sufficient retention of wet strength over the period of intended use.
It is an object of this invention to provide improved temporary wet strength resins that can provide high levels of initial wet strength and which retain sufficient strength during the period of intended use, but which also facilitate wet strength decay such that very low strength levels are attained subsequent to the period of intended use.